Along with the recent miniatuarization and high functionalization of electronics, a printed board used for mounting a semiconductor or testing has been increasingly required to permit fine and high density wiring. As a result, a hole formed for providing a protruding contact point and the like as a probe on a printed board needs to be fine and formed at high densities.
In particular, fine holes having a diameter of not more than 100 .mu.m are advantageously formed by laser ablation. As the light source for such hole-forming, an excimer laser is particularly preferable. The excimer laser has an oscillation wavelength of not more than 400 nm, and laser ablation using said laser for resin decomposition enables formation of fine holes having a diameter of not more than 100 .mu.m with high precision.
On the other hand, laser ablation is uneconomical due to its high processing cost caused by the long time of processing time necessary for forming holes and low energy utilization efficiency, as explained in the following.
FIG. 5 shows an embodiment of conventional processing, wherein a laser beam 2 is output from a light source 1 (excimer laser), passes through a scanning mirror 14, a mask 4 and an imaging lens 6, and hits a printed board 7 to form a through-hole. Such a method for forming a hole (inclusive of through-hole and dead-end hole) which comprises shooting a laser beam through the mask 4, the mask 4 having light path holes 5 for the passage of the light, which correspond to the holes to be formed in the printed board 7, wherein the laser beam 2 which passed the light path holes 5 ablates the printed board 7 and forms holes is a mask imaging method.
As shown in FIG. 6 wherein conventional laser ablation is used, when the entire pattern 13 of the arrangement of the light path holes 5 formed in a mask 4 is greater in size than the sectional area 12 (the size of the section perpendicular to the advance direction of said beam, namely, the size of irradiation area) of the laser beam, the laser beam needs to be moved for scanning.
For example, the laser beam irradiation area should be moved to sequentially follow the pattern 13 of the light path holes 5 (e.g., route 15 shown by arrows in the embodiment of FIG. 6), so that the laser beam successively passes the light path holes 5. In the embodiment of FIG. 5, for example, the scanning mirror 14 is moved to change the irradiation area of the laser beam 2 on the mask. Consequently, some time is necessary for completing entire holes, thus increasing the processing cost.
As shown in FIG. 7, moreover, when the entire pattern 13 of the arrangement of the light path holes 5 is smaller in size than the sectional area 12 of the laser beam, the total sectional area (area of the section perpendicular to the axis of the hole) of the light path holes 5 becomes much smaller than the sectional area of the beam, so that only a part of the energy of the laser beam output from the light source passes the holes, wasting the light prevented by the mask. Consequently, this method is uneconomical with low energy utilization efficiency.
For forming a probe, Japanese Patent Unexamined Publication No. 2-129938 proposes a method for producing a circuit board, which comprises forming, by laser processing, fine through-holes in an insulating resin film having a conductive circuit in the thickness direction of said insulating resin film, filling the through-holes with a metallic material to be a contact material by a method such as plating, and forming a bump electrode by allowing further protrusion.
When the hole in which a bump electrode is to be formed is provided by various laser beams having a relatively high output power, such as CO.sub.2 laser, YAG laser and excimer laser, the residue produced thereby imposes a different problem.
For example, when a through-hole is formed in a resin layer by CO.sub.2 laser, YAG laser and the like and a conductive circuit is exposed on the inner bottom of said hole, a slight amount of a processing residue is produced on the exposed conductive circuit, as shown in FIG. 8(b), due to which a bump electrode cannot be formed.
In the ablation processing using excimer laser beam, the material of the resin layer which absorbed the laser beam is decomposed and diffused, and, as shown in FIG. 8(b), adheres not only to the inside of the through-hole but to the circumference of said hole as a decomposition residue. When the decomposition residue is present around the hole during the growth of a bump electrode, the main component, carbon, causes unintended growth of the bump electrode extending from the hole in the planar direction of the resin layer to result in short circuit between neighboring bump electrodes and variation in height and outer diameters of the bump electrodes, thus resulting in possible occurrence of defective products.
The processing residue and decomposition residue produced by the above-mentioned laser processing cannot be removed sufficiently by a mechanical removing means using a brush and the like, immersion in water or organic solvent, ultrasonic vibration and the like. In particular, when the center pitch or diameter of the through-hole is not more than several dozen .mu.m, this problem becomes prominent.
It is therefore an object of the present invention to provide a method for forming a hole by laser ablation capable of forming a hole in the resin layer of a printed board at a higher speed with an improved energy utility efficiency, while producing less amount of residue.
It is another object of the present invention to provide a method for removing a residue remaining inside and in the circumference of the hole provided in the resin layer of a printed board, by adapting the above-mentioned processing method of the present invention.